Nowadays, the scale of the Internet is expanding at an amazing speed along with the fast development of mobile communications. More and more mobile subscribers expect to access the Internet more flexibly without being restricted by time or space. To meet the requirement, a Mobile Internet Protocol (MIP) technology emerges. MIP is a new interconnection technology that supports the connection between a mobile subscriber and the Internet, and enables the mobile subscriber to move to another place without interrupting the ongoing Internet communication. MIP becomes a hot topic in the industry.
MIP allows a terminal to use two Internet Protocol (IP) addresses. The first address is a Home Address (HoA), which is fixed; and the second address is a Care-of Address (CoA), which changes at every new connection point. MIP allows a computer to roam freely on the Internet and on the intranet of an organization, without changing the HoA. Therefore, when the terminal changes the connection point of the computer, the communication goes on, and MIP updates the network according to the new location of the terminal.
MIP may route an IP packet to each connection point. Wherever the terminal is connected, the HoA of the terminal always identifies the terminal. If the terminal is outside the home network, the CoA needs to be associated with the HoA of the terminal. The CoA may provide the information about the current connection point of the terminal. The terminal may use an agent notification to detect when it moves from one subnet to another. When the terminal receives an agent notification indicating that the location has changed, the terminal performs registration through a foreign agent. FIG. 1 illustrates how a terminal performs MIP registration with a home agent through a foreign agent. First, the terminal sends a registration request to a future foreign agent, thus starting the registration procedures. After processing the registration request, the foreign agent relays the request to the home agent. The home agent sends a registration response to the foreign agent to accept or reject the request. After processing the registration response, the foreign agent relays the response to the terminal, indicating the state of processing the request.
The connection point addresses are allocated through a Dynamic Host Configuration Protocol (DHCP). As shown in FIG. 2, the allocation process includes four stages: discovery stage, in which the DHCP client searches for the DHCP server; provision stage, in which the DHCP server provides an IP address; selection stage, in which the DHCP client selects an IP address provided by a DHCP server; and confirmation stage, in which the DHCP server confirms the provided IP address.
Currently, the Proxy Mobile IP (PMIP) protocol derived from MIP is also a mobile technology protocol. In PMIP, the terminal does not provide the MIP function. Many of MIP functions are implemented by a foreign agent that has the PMIP client capabilities. The PMIP client needs to participate in both exchanging the MIP registration messages and allocating an IP address to the terminal, as shown in FIG. 3. The terminal broadcasts a DHCP Discover message. A DHCP relay intercepts the DHCP Discover message, and forwards the message to the DHCP server according to the configuration information. After receiving the DHCP Discover message, the DHCP server sends a DHCP Offer message to the terminal. The provided IP address is carried in this DHCP Offer message. After the terminal receives the DHCP Offer message, if the terminal selects this IP address, the terminal sends a DHCP Request message to the DHCP server. After receiving the DHCP Request message, the DHCP server sends a response indicative of IP address allocation success to the terminal. After receiving the response, the DHCP relay prompts the PMIP client to initiate MIP registration procedures; that is, the PMIP client sends a MIP Registration Request (MIP Reg Req) to the home agent. After receiving a MIP Registration Response (MIP Reg Res), the PMIP client triggers the DHCP relay to forward the DHCP Response message to the terminal.
However, the inventor of the present invention discovers that the existing DHCP relay is capable of forwarding only, and is unable to send a message initiatively or change the type of message optionally. Therefore, when receiving the information about MIP registration failure, the DHCP relay is unable to instruct the DHCP server to reclaim the address because it is unable to send messages initiatively; in addition, the DHCP relay is unable to convert an Acknowledgement (ACK) message into a Not Acknowledgement (NAK) message which is to be notified to the terminal, where the ACK message is sent from the DHCP server and indicative of IP address allocation success. Consequently, the terminal is unable to know unavailability of the address, and the transmission of subsequent messages of the terminal fails.